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Copyright © Gabor Fichtinger, 2008Laboratory for Percutaneous Surgery – The Perk Lab

XX--ray imagingray imaging


WWüürzburgrzburg, November 8, 1895: Dr. , November 8, 1895: Dr. Wilhelm Conrad Wilhelm Conrad RRööntgenntgen


Heat

=DC

Fluorescence in glass tube by cathode Fluorescence in glass tube by cathode raysrays

- ----

- -

Glowing light in gas


RRööntgenntgen’’ss experimentexperiment

Heat

=DC

- ----

- -

Black cardboard box

Faint glowing

Barium Platinum Cyanide

50cm


What did What did RRööntgenntgen know and did not know?know and did not know?

DID KNOW• Ray-like phenomenon (named it X for “unknown”

ray)• Not charged• Carries to some distance• Penetrates through regular materials• Absorption rate depends on material density• Absorption depends on material mass• Carries significant energy• Energy changes with tube voltageDID NOT KNOW• Electromagnetic wave, just like visible light• Shorter wavelength (E=h*ν)• Always harmful, potentially lethal


Frau Frau RRööntgenntgen’’ss hand, December, 1895hand, December, 1895


XX--ray of a handray of a hand1895

2002


••

•

Modern chest XModern chest X--ray machineray machine

Number of X-rays registered by Medicare in 1993:

207,753,747


XX--ray process (in its full glory)ray process (in its full glory)


Silver bromide film coating Silver bromide film coating


Chest XChest X--ray, 1903ray, 1903


Army hospital ~1915, waiting for XArmy hospital ~1915, waiting for X--rayray


Bucky Grid (1913)Bucky Grid (1913)


PotterPotter’’s Grid (1920)s Grid (1920)


Collimator gridCollimator grid


XX--Ray dermatitis & protective garmentRay dermatitis & protective garment

19101910 19141914


XX--ray interaction with matter ray interaction with matter –– ionizing ionizing radiationradiation


Figure 24. An important, and adjustable, determinant of image quality in radiography is the voltage applied across the X-ray tube by the generator. The image to the left, (a), was made at a lower voltage (70,000 V) than (b), the one on the right (110,000V). As a result, (a) more clearly shows the two ureters, which are carrying urine (and iodine-based contrast agent) from the kidneys to the bladder. The lowerthe voltage, the better the contrast, but the greater the amount of radiation dose that is delivered to the patient-one of many trade-offs to be considered in imaging. Courtesy of the American College of Radiology.

Effect of tube voltageEffect of tube voltage


Exponential decay of intensity in Exponential decay of intensity in mattermatter


XX--ray interaction with DNAray interaction with DNA


Radiation risk assessmentRadiation risk assessment

• Risk = possibility that something bad may happen• Public’s feeling depends on source of risk

– Human made hazards are less acceptable– Risk feels greater if comes without consent or consultation– Accidents increase the feel of risk

• In radiology: possibility that ionizing radiation used in medical procedures might give rise to cancer

• It has been documented to happen• Risk = Dose * Riskprobability• Riskprobability depends on the type of cancer & organ• Dose is what we can control…• Yardstick: whole-body background 300 mrem /year• Chest X-ray: 20 mrem at skin, 1-2 mrem at exit• Equivalent cancer risk:

– Skin : cosmic backgr = 15 shots /year– Internal organs: cosmic backgr = 100-150 shots /year


Dose escalation (accumulation) Dose escalation (accumulation) modelsmodels

• Under debate• It has been linear accumulation model = the body never forgets

radiation• But the body does forget radiation, we just do not know how.• How does genetic distortion caused by ionizing radiation fades

out? – we do not know• New: “annual limits” for medical staff• Dose tolerance levels are set for organs• Sensory organs are very sensitive• Brain, lymph nodes, bone marrow are generally sensitive• Large differences in tolerance, even inside brain


XX--ray = terror and Xray = terror and X--ray = horrorray = horror


The geometry of XThe geometry of X--ray ray

X-ray film

X-ray source (point source)

beam central

Object

Shadow

SFD (source-film distance)

STD (source-targeplane dist)

Linear magnification=

Areal magnification =

SFD/STD

(SFD/STD)2

Object plane


The fundamental problem of XThe fundamental problem of X--ray ray

X-ray film


Object ?

Shadow

From the X-ray image only, we cannot tell the location, shape and size of the target

Because:• Projection has no inverse• Projection is irreversible• Projection loses sense of

depth


Forward projection Forward projection

X-ray film


Object

Image

Project an object onto the Project an object onto the film/detector to obtain an film/detector to obtain an image.image.


Are depth, 3D shape and volume lost forever?Are depth, 3D shape and volume lost forever?


3D coordinate space3D coordinate space

Source-1 Source-2

X-ray film-1

X-ray coordsystem

Film-1 coordsystem

Film-2 coordsystem

X-ray film-2


Back projection Back projection

Plane of the X-ray film


Object ?

Image on film

Beam central

?

?

?

Draw a line from the Draw a line from the centercenter of the image of the image

backback from film to the Xfrom film to the X--ray source.ray source.

The object must sit on The object must sit on this line somewhere.this line somewhere.


Can we find (segment) the center Can we find (segment) the center accurately?accurately?

++


Reconstruction from 2 filmsReconstruction from 2 films

X-ray film #2

X-ray film #1

Find the intersection of the two back-projection line


• We look for the intersection of lines in 3D space• Mathematical trouble – the intersection generally does

not exist

• The two back projection lines avoid each other

• Why?

– Image processing errors

– Measurement errors

– Calibration errors

– Mechanical imperfections

Solve for the intersection of backSolve for the intersection of back--projection projection lineslines


EpipolarEpipolar constraintconstraint

film #2

film #1

Back-project image from film1. Then forward project this back-projection line onto film2. The result is a line on film2. Image2 must lay on this line (or close to this line).

forwardforward

back


Pros and cons of plain XPros and cons of plain X--rayray• Pros:

– Simple– Inexpensive– Excellent bone contrast

• Cons:– Harmful dose to patient– Loss of shape and depth of objects– Poor soft tissue contrast– Delay between shots and seeing films

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